Aligned with the National Board of Medical Examiners format. Authored & peer-reviewed by faculty, clinicians, and clerkship directors.
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A 6-year-old girl, who had been camping in Vermont the week prior, is admitted from the ED after 5 days of fever, headache, and myalgias. On the day of admission, she developed a widespread maculopapular rash on her torso and extremities, including the palms and soles. Physical exam reveals an ill-appearing girl with intermittent confusion and hepatosplenomegaly. Laboratory findings include leukopenia, thrombocytopenia, hyponatremia, and elevated transaminases. Which one of the following tests is most likely to lead to the diagnosis at this time?
This patient has symptoms consistent with human monocytic ehrlichiosis, a tick-borne infection spread by Amblyomma americanum, or lone star ticks. The most commonly responsible organism, Ehrlichia chaffeensis, is a gram-negative, small, obligate intracellular bacterium. States with the highest incidence of human monocytic ehrlichiosis in recent years include New Hampshire, Vermont, Rhode Island, Delaware, Virginia, Kentucky, Tennessee, Missouri, Arkansas, Kansas, and Oklahoma. Most cases are seen predominantly in May through September, largely because there are higher numbers of lone star ticks during this time. Symptoms typically present between 2 and 21 days after the tick bite, although approximately one-fourth of patients have no history of a known tick bite. Fever, headache, and myalgias are the most frequently reported symptoms. The majority of children develop a rash, and about one-half develop organomegaly and lymphadenopathy. Altered mental status may also be present, and other symptoms may include nausea, vomiting, or abdominal pain. Laboratory evaluation often reveals leukopenia, thrombocytopenia, hyponatremia, and elevated transaminases, as this patient has. During the acute phase of illness, PCR amplification of Ehrlichia chaffeensis is the most sensitive diagnostic method, as antibody titers are often negative. However, a comparison of acute and convalescent IgG antibody titers by indirect immunofluorescence assay may be diagnostic if there is a fourfold rise.
ELISA for Lyme disease (A), with a reflex western blot test, if positive, is useful to confirm the diagnosis in a patient whose clinical features are suggestive of Lyme disease. Also a tick-borne infection, Lyme disease often presents with erythema migrans rash or joint swelling in children. The heterophile antibody test (B) is useful for diagnosing infectious mononucleosis, particularly in the second week of illness. Infectious mononucleosis is most commonly caused by the Epstein-Barr virus. This patient’s symptoms are similar to those seen with Rocky Mountain spotted fever. However, its incidence in Vermont is low when compared to that of human monocytic ehrlichiosis. Additionally, an IgG antibody titer for Rocky Mountain spotted fever (C) is unlikely to be helpful in the acute phase of the illness. Similar to antibody titers for ehrlichiosis, a comparison between acute and convalescent titers is necessary for diagnosis.
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Ehrlichiosis and Anaplasmosis
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